It is known for a gas turbine engine to have two independently controlled sets of burners each set containing a plurality of burners, the two sets may be main burners and pilot burners. The pilot burners are arranged in parallel to receive the fuel from a pilot burner manifold, and one or more groups of main burners, the burners of each group receiving fuel from a respective main burner manifold.
It is known to supply fuel to the pilot burners through a Pressure Raising Valve (PRV) which ensures a minimum system pressure sufficient to drive various valves upstream of the PRV. The PRV receives a reference pressure against which it controls the pressure in the flow line to the pilot burners. Conventionally the reference pressure for the PRV is the low pressure fuel supply to the fuel system. It will be well understood by those skilled in the art that devices through which fuel flows, such as valves and burners in the fuel system, have a characteristic which is known as the xe2x80x9cflow numberxe2x80x9d. The flow number is representative of the flow area of the component, and thus in simple terms can be considered to be a measure of the maximum flow capability of the component. In the case of main and pilot burners their flow numbers may well be such that with all valves fully open 70% of the fuel would flow through the main burners and 30% of the fuel would flow through the pilot burners. The main and pilot burners would then be said to exhibit a 70/30 flow split. It can be shown (as will be explained in more detail hereinafter) that in a conventional system having a ratio of main/pilot burner flow numbers of 70:30, although the flow of fuel through the burners can be varied to control both the total fuel supplied to the burners and the split of that total fuel flow between the main and pilot burners, a main/pilot flow split in excess of 70/30 cannot be achieved. It is an object of the present invention to provide a fuel system wherein the control mechanism can achieve a main/pilot flow split exceeding the main/pilot burner flow number ratio.
In accordance with the present invention there is provided a fuel system for a gas turbine engine having a main burner fuel supply line and a pilot burner fuel supply line, a high pressure pump supplying fuel to the main and pilot burner supply lines in use, a throttle valve controlling the flow of fuel in the main burner supply line, a metering valve controlling the flow of fuel in the pilot burner supply line, and a pressure raising valve between the pilot burners and the metering valve, the pressure raising valve serving to maintain the system fuel pressure upstream of the pressure raising valve to a pre-determined amount in excess of the pressure of fuel supplied from the throttle valve to the main burners.
Preferably said pressure raising valve has a movable valve member exposed to fuel pressure in a reference chamber of the valve, and said reference chamber is connected in use to a fuel line from said throttle valve to said main burners whereby the pressure in said reference chamber is the pressure of fuel supplied from said throttle valve to said main burners.